Method for synchronizing an equalizer output data

ABSTRACT

A method for synchronizing symbols at the output of a blind equalizer, the method being characterized by the following steps: on sending, inserting into a succession of sent symbols, one or more known sequences of symbols repeated in said succession of symbols; detecting said one or more known sequences at the output of said blind equalizer; deducing any shifting of the symbols at the output of the blind equalizer from the result of said detection; and retiming the symbols at the output of the blind equalizer as a function of the deduced shift.

GENERAL FIELD AND PRIOR ART

The present invention relates to a method for synchronizing an outputdata from a blind equalizer.

Digital communications systems conventionally use receivers thatinclude, in cascade, demodulator means, equalizer means, decoder means,etc.

The function of the equalizer means is to combat intersymbolinterference caused in particular by the presence of multiple paths thatare static or non-static over time.

Many kinds of equalizer means are already known in the art.

Historically speaking, the first systems used to combat intersymbolinterference were essentially “synchronous” linear transversal filters.

Other equalizer structures that have been used include recursivenon-linear decision feedback equalizers (DFEs) in which data that hasbeen decided on is injected into a backward filter constituting therecursive portion of the equalizer.

The above kind of equalizer structure is generally used for transmissionchannels with adaptation algorithms for adjusting the parameters of thestructures, in which case equalization is carried out in two stages.During a first stage, the structure is controlled by known trainingsequences that are inserted into the frames that are sent and cause theequalizer algorithm to converge. During the second stage, the structurebecomes self-adaptive, i.e. it controls itself on the basis of its owndecisions.

However, using a training stage has a serious drawback; in particular,it corresponds to a loss of efficiency in terms of bit rate.

For this reason considerable research has already been conducted with aview to proposing blind equalizer systems whose structure is able toconverge towards an optimum solution in a “self-trained” manner, i.e.without using training sequences.

Blind equalizers that use different adaptation algorithms and structuresaccording to the severity of the transmission channel have recently beenproposed in patent application FR 2 738 967 and in a paper by J. Labat,O. Macchi and C. Laot “Adaptive Decision Feedback Equalization: can youskip the training period?”, IEEE Transactions on Communications, Vol.46, No. 7, July 1998.

In particular, when reception is difficult, those equalizers use aconvergence structure that includes, in cascade, a purely recursivefilter and a transversal filter, whereas when reception is easy, theyfunction in a tracking mode that uses conventional DFE structures, thedecision to switch from one mode of operation to the other being afunction of the performance achieved at the equalizer output.

It should be noted that the reversible nature of the change from one ofthe above two modes of operation to the other means that thoseequalizers can always function with a configuration that corresponds tooptimum performance. They can therefore operate according to their owndecisions with no risk of divergence, unlike conventional DFE. Thisessential property enables them to adapt to severe channel fluctuations,and therefore makes them particularly suitable for non-stationarychannels, such as mobile radio channels, ionospheric channels, andsubmarine acoustic channels.

SUMMARY OF THE INVENTION

First tests on blind equalizers have shown loss of timing phenomena thatcan be blamed on the adaptive nature of the equalizers, which, in thepresence of a plurality of echoes in the received signal, do not alwaystend to adapt to the same echo, and then switch from one echo toanother.

This results in lags or leads in the reproduction of symbols, i.e. tosymbols being eliminated from or added to the stream of data.

This is illustrated by the following example.

H is a channel characterized by its impulse response as described by amatrix H such that:H=[h ₀0 . . . 0 h ₁₆ 0 . . . 0]^(T)

The symbols sent and received satisfy the following equation, in whichd(n) represents the symbols sent and r(n) represents the symbolsreceived:r(n)=h ₀ ·d(n)+h ₁₆ ·d(n−16)

At the start of transmission, the data is received on a first channel C1corresponding to h₀=1 and h₁₆=0.

Since the main send/receive path varies during transmission, theequalizer switches to another channel corresponding to h₀=0 and h₁₆=1,for example (i.e. to the channel C2, corresponding to the second path,which has become the main path).

The table below shows that considering the second path as the main pathleads to repeating the data d(4) and the subsequent data at the outputof the equalizer and therefore to the creation of a non-zero error e(n).TABLE 1 Example of shifting of data N 16 17 18 19 20 21 22 23 Ch. C₁ C₁C₁ C₁ C₂ C₂ C₂ C₂ d(n) d(16) d(17) d(18) d(19) d(20) d(21) d(22) d(23)r(n) d(16) d(17) d(18) d(19) d(4) d(5) d(6) d(7) e(n) ≈0 ≈0 ≈0 ≈0d(4)-d(20) d(5)-d(21) d(6)-d(22) d(7)-d(23)

The change from the channel C1 to the channel C2 leads to lags in thereproduction of the symbols and therefore to the addition of symbols;conversely, switching from the channel C2 to the channel C1 leads toleads in the reproduction of symbols and therefore to the elimination ofsymbols.

The object of the invention is to alleviate this drawback and to proposea method of limiting the effects of losses of timing encountered in thefunctioning of blind equalizers.

To this end, the invention proposes a method of synchronizing data atthe output from a blind equalizer, the method being characterized by thefollowing steps: inserting synchronization sequences into the frames ofsymbols; detecting these sequences at the output from the equalizer; andretiming the frames of symbols as a function of the shift detected inthese sequences.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention come to light in thecourse of the following description, which is purely illustrative, isnot limiting on the invention, and is given with reference to theaccompanying drawings, in which:

FIG. 1 represents a digital communications transmission channel,

FIG. 2 represents signal processor means used at the output of thattransmission channel in the context of the invention,

FIGS. 3 a, 3 b and 3 c represent the loss of timing phenomenon and anexample of data synchronization at the output of the blind equalizer,

FIG. 4 represents the detection of shifts generated by losses of timing,and

FIGS. 5 a and 5 b represent a blind equalizer structure used inconvergence mode and in tracking mode, respectively.

DESCRIPTION OF ADVANTAGEOUS EMBODIMENT(S) General Description of oneEmbodiment

FIG. 1 shows an input transmission channel over which successions ofsymbols d(n) are sent.

The channel is symbolized by a transfer function h(t) and by noise b(n)added at the output of the transfer function.

At the output of this transmission channel, the resulting symbols r(n)are received and processed in a receiver that includes in particularmeans of the type represented in FIG. 2.

These means include in particular a blind equalizer 1 which receives atits input the symbols r(n), which are filtered beforehand, whereapplicable, and processor means 2 whose function is to process the datay(n) at the output of the equalizer 1 to limit the effect thereon of theloss of timing phenomenon.

The processing effected by the processor means 2 consists in:

-   -   detecting, in the frames of symbols at the output of the blind        equalizer 1, known sequences SYNCH inserted into the frames of        symbols d(n) at regular intervals,    -   deducing any shift in the symbols processed by the equalizer 1,        and    -   retiming the data at the output of the equalizer as a function        of the shift determined in this way.

This is represented in FIGS. 3 a to 3 c in particular.

FIG. 3 a represents a succession of frames corresponding to symbols r(n)received by a receiver and sent to the input of the blind equalizer 1.

The received frames include the known sequences SYNCH at regularintervals.

FIG. 3 b illustrates a loss of timing introduced by the blind equalizer1.

Here the loss of timing is a loss that leads to adding symbols,reflected in the repetition of the symbols S_(k−17) to S_(k).

The symbols after and including the symbol S_(k+1) are shiftedaccordingly, with the result that the sequence SYNCH that follows theloss of timing appears in the output data y(n) of the equalizer 1 at atime D instead of at an expected time C.

Correlating the data y(n) with the sequence SYNCH indicates the time Dat which said sequence arrived and this enables the shift CD to beestimated.

FIG. 3 c shows that, in order to retime the sequences SYNCH at theoutput of the equalizer, the succession of symbols is shortened orlengthened between the sequence SYNCH for which a shift (and thus a lossof timing) is detected and the preceding sequence SYNCH.

For example, in the situation represented in FIGS. 3 a to 3 c of a lossof timing that leads to adding symbols, the succession of symbols isshortened between the two sequences SYNCH by eliminating a number ofsymbols corresponding to the estimated shift after the sequence SYNCHthat precedes the sequence SYNCH for which a shift is detected.

Of course, other solutions could be envisaged: in particular,eliminating symbols situated just ahead of the sequence SYNCH for whicha shift is detected could be envisaged.

Example of Retimer Means

For the purposes of the retiming process that has just been described,the FIG. 2 processor means 2 include a synchronization filter 3 and ashift management unit 4.

The filter 3 and the management unit 4 each receive at their input thesignal y(n) at the output of the blind equalizer 1.

The filter 3 applies continuous correlation with the synchronizationsequence and supplies at its output a signal of the type represented inFIG. 4.

That signal has peaks at times that correspond to the synchronizationsequences SYNCH at the output of the equalizer.

The output of the filter 3 is normalized with respect to the power ofthe samples present in the filter.

Here a correlation peak is looked for in a frame equal to that of thesent data.

As can be seen in FIG. 4, a peak is detected in each frame, for exampleby searching for a maximum. The position of the peak relative to areference time is used to detect any shift relative to the previouspeak, and that shift is used to determine if a loss of timing hasoccurred.

To be more precise, the peak is detected by comparison with a giventhreshold, for example 0.7.

No shift is applied to the symbols y(n) at the output of the equalizerif no peak is detected, which is the case in particular if two pathshave the same power or if the signal-to-noise ratio is particularly low.

The shift management module 4 shifts the output data stream in onedirection or the other according to information concerning the lag inreproducing symbols obtained at the output of the synchronization filter2. That shift in one direction or the other may easily be effected bymeans of a buffer to which the output data of the blind equalizer 1 issent, for example. The size of the buffer is determined as a function ofthe maximum lag to be managed.

Note that the synchronization filter 3 and the correlation of the levelof said filter with the synchronization sequence SYNCH areadvantageously used to manage the phase shifting of the received signalin parallel with resolving phase ambiguity in the equalizer outputsymbols.

As shown in FIG. 2, the phase shift found by the filter 3 is thereforeinjected into the data y(n) at the output of the retimer module 4.

Synchronization Sequences

The synchronization sequences SYNCH are those used as training sequencesif the receiver includes interference canceling means on the downstreamside of the equalizer, for example.

Note that the size necessary for the synchronization sequences is lessthan the size necessary for the training sequences of equalizers thatuse training sequences.

For example, the synchronization sequences can occupy only some 30% ofthe frames, or even less.

One possible synchronization sequence is the following 31-bitpseudorandom sequence, for example:

-   [0000101011101100011111001101001]    which corresponds in this embodiment, and in the case of an MDP4    modulated signal, to the following succession of symbols:-   [00 00 00 00 11 00 11 00 11 11 11 00 11 11 00 00 00 11 11 11 11 11    00 00 11 11 00 11 00 00 11].    Blind Equalizer Example

FIGS. 5 a and 5 b represent one possible structure of the blindequalizer.

The context is that of a self-adaptive decision feedback equalizer (SADFE) with a suitable structure and a suitable algorithm of the typedescribed in the paper and the patent application cited above.

As indicated above, this kind of equalizer has two modes of operationadapted to the severity of the transmission channel.

In an initial mode, called the convergence mode, the device comprises,in cascade, a purely recursive whitening filter B(z), a transversalfilter A(z), an automatic gain controller, and a phase corrector. Thecriteria for updating the coefficients of the transversal and recursiveportions are based entirely on an a priori knowledge of the statisticsof the signal sent by the source. This initial mode is thereforeperfectly “self-trained” (i.e. blind or “non-supervised”). FIG. 5 arepresents the structure of the SA DFE in the convergence mode.

When the equalization process is sufficiently advanced, which may bedetermined from the mean square error estimated from the decisions takenby the receiver, the structure and the adaptation criteria of theequalizer are modified so that the device switches to the tracking mode,in which the settings of the transversal and recursive filters aremodified to convert the device into a conventional decision feedbackequalizer (DFE). The condition for optimum overall functioning thenbecomes that of a minimum estimated mean square error. FIG. 5 brepresents the structure of the SA DFE in the tracking mode.

It is therefore clear that the equalizer has two different modes ofoperation associated with different structures and different criteria ofoptimum functioning. One essential feature of the equalizer is that thisstructural modification is totally reversible. This is beneficialbecause it allows reversion to a very robust mode of operation in severesituations. On the other hand, as soon as the channel becomes lesssevere, the system switches back to the tracking mode.

During the convergence phase, the coefficients of the filter A(z) areinitialized to 0 except for one coefficient that is initialized to 1.That coefficient is positioned so that the filter approximates ananti-causal structure, that is to say, in the present example, towardthe right.A(z)=[0, 0 . . . 0, 1, 0, 0, 0, 0]^(T)

The coefficients are then adapted using a constant modulus algorithm(CMA). For more details of CMA see Zhi Ding, Ye Geoffrey Li, “BlindEqualization and Identification”, Signal Processing and CommunicationsSeries, 2001.

Reinitialization

Note that if the channel does not vary in time, the position of thecoefficient equal to 1 determines the constant shift at the output ofthe blind equalizer to be taken into account.

Because the channels do vary in time here, this coefficient is caused toshift, and as soon as it shifts too far toward a causal structure (i.e.toward the left in the present example), the performance of theequalizer is degraded, as explained in the above publication by Zhi Dinget al.

In order for the performance of the equalizer not to be degraded thecoefficients of the filter A(z) are then regularly reinitialized inaccordance with an anti-causal structure in the manner described above.

This reinitialization can also lead to losses of timing.

In this case, the symbols are retimed using the synchronizationsequences SYNCH.

1. A method for synchronizing symbols at the output of an equalizer,characterized in that the equalizer is a blind equalizer and that themethod comprises the following steps: on sending, inserting into asuccession of sent symbols, one or more known sequences of symbolsrepeated in said succession of symbols, detecting said one or more knownsequences at the output of said blind equalizer, deducing any shiftingof the symbols at the output of the blind equalizer from the result ofsaid detection, and retiming the symbols at the output of the blindequalizer as a function of the deduced shift.
 2. A method according toclaim 1, characterized in that, to detect a known sequence inserted, onsending, into a succession of symbols, the symbols at the output of theequalizer are correlated with said sequence and the resultingcorrelation peaks are detected.
 3. A method according to claim 2,characterized in that detected correlation peaks are compared to a giventhreshold and the symbols are not retimed unless a peak higher than saidthreshold is detected.
 4. A method according to claim 2, characterizedin that the result of said correlation is used to determine informationon the phase of the signal carrier that carries the received symbols andthat information is used to resolve ambiguity as to the phase of thesymbols at the output of the equalizer.
 5. A method according to claim1, characterized in that, to retime frames, symbols are eliminated fromor added to the succession of symbols at the output of the equalizerbetween the sequence for which a shift is detected and the precedingsequence.
 6. A method according to claim 5, characterized in thatsymbols are eliminated just after the sequence preceding the sequencefor which a shift is detected.
 7. A method according to claim 1,characterized in that the blind equalizer has a switchable structure,uses a switchable algorithm, and, in a convergence mode of operation,includes in cascade a purely recursive whitening filter and a matchedtransversal filter that is reinitialized as a function of theperformance of the equalizer.
 8. A digital communications receiverincluding a blind equalizer, characterized in that it includes means fordetecting, at the output from said blind equalizer, a sequence insertedinto a succession of received symbols and means for deducing from theresult of said detection any shifting of the symbols at the output ofthe blind equalizer and means for retiming the symbols at the output ofthe blind equalizer as a function of the shift detected.
 9. A receiveraccording to claim 8, characterized in that it includes aturboequalization system of which the blind equalizer is a first stage.10. A receiver according to claim 8, characterized in that it includesan interference canceling stage on the downstream side of the blindequalizer and the known sequences used to retime the symbols at theoutput of the blind equalizer are sequences also used for training saidinterference canceling stage.